Lipoprotein lipase (LPL) is the key enzyme involved in normal triglyceride metabolism. In the presence of its cofactor, apoC-II, LPL hydrolyzes triglycerides present in chylomicrons and VLDL to mono- and di-glycerides and free fatty acids. The genetic defects that lead to a deficiency of apoC-II have been elucidated in 3 different kindreds with the familial hyperchylomicronemia syndrome. A single base pair mutation results in the introduction of a stop codon in the signal peptide of the apoC-II(paris2) gene. Two independent mutations have been identified in the apoC-II and apoE genes of the proband from a Spanish kindred which result in a combined functional deficiency of apoC-II and apoE. The identification of 2 independent mutations occurring in 2 genes which are closely linked is unique. The proband from the third kindred is a compound heterozygote with 2 different allelic mutations in the LPL gene that lead to the synthesis of non-functional LPL. Expression studies have established the functional significance of these mutations. We have previously described a mutant LPL (LPL(Bethesda)) with an ala176, to the substitution near the interfacial recognition binding site of LPL that leads to both abnormal LPL activity and heparin binding. In order to understand the role that alal76 plays in maintaining normal LPL function we have modified the residue by site-directed mutagenesis. Substitution of alal76 by any residue except for gly results in the loss of LPL activity suggesting that stearic hindrance plays a role in the loss of activity of LPL(Bethesda). Substitution of serl32 by gly leads to a total loss of hydrolytic activity consistent with its role as the catalytic serine. The luciferase reporter gene was used to analyse the regulatory elements which modulate the expression of apoC-II in liver. A 5' flanking 542 bp fragment contains the control elements that direct tissue specific expression of apoC-II. Deletion studies established that apoC-II expression is modulated by proximal and distal cis-acting elements and that the region from -130 to +10 is sufficient to achieve maximal levels of gene expression.